The present invention relates to a system filter for double frequency utilization, including a broadband polarization filter in conjunction with frequency filters, for separating two different frequency bands where each frequency band is itself doubly orthogonally polarized.
The desire for a high number of channels in data transmission systems, particularly for transmission via satellites and in radio systems, requires optimum utilization of the instruments involved and of the antenna arrangements. For this purpose the use of polarized systems offers itself automatically since in this way two channels can be used at the same frequency. However, a prerequisite for this use is very high decoupling of the two polarized signals which then permits genuine double frequency utilization.
Since usually two frequency bands which are spaced relatively far apart are required for transmitting and receiving (e.g. 11 and 14 GHz bands), these antenna multiplex systems take on the function, in addition to separating the two polarizations, of being frequency filters. A combination of a polarization filter and a frequency filter is called a system filter.
German Pat. No. 1,128,491 discloses a classical polarization filter of the type utilizing a waveguide to whose outputs frequency filters can be connected. This filter is suitable only for small spacings between the frequency bands since the occurrence of higher waveguide modes constitutes an upper limit for its use under the above-mentioned conditions.
The Siemens advertising pamphlet No. S. 42024-04-A-2-29 "System filter 4/6 GHz" describes a filter arrangement with frequency selective 0 dB couplers. These filters are used, for example, to couple the two polarizations of the higher frequency band out of a square or circular waveguide by means of highpass directional couplers, while the polarization separation in the low frequency band is effected by means of a classical polarization filter. This filter is also suited for frequency bands which are spaced far apart (e.g. 4 and 6 GHz, 11 and 18 GHz) but it has the drawback that losses are relatively high and its external dimensions are rather large.
A doubly symmetrical polarization filter operating with four decouplers is disclosed in applicant's copending allowed U.S. Pat. application Ser. No. 611,974, filed Sept. 10, 1975, now U.S. Pat. No. 3,978,434, issued Aug. 31, 1976. This filter principle can be utilized, depending on the particular embodiment, for frequency bands which are spaced more or less far apart (e.g. 12/18 or 11/14 GHz). The drawback of this filter arrangement is, however, that electrical asymmetries in the four decoupling branches may lead to cross coupling of the polarizations.